This is a rattle made of bronze, called a sistrum. When
waved about, the bars rattle loudly together. It was used in the
worship of the goddess Isis.

The Romans worshipped many different gods based on nature, and
as they conquered new lands they sometimes borrowed gods from these
countries. Rome conquered Egypt in 30BC. Isis was an important
goddess from ancient Egypt who was responsible for the family, and
she became a popular Roman goddess. There was a temple to Isis in
every major Roman city, including Pompeii, where wall paintings
show priests holding rattles just like this one.